Why Are Some Michigan Roads Known as ‘Trunk Lines?’

Numbered highways in Michigan are officially known as 'Trunk Lines.' How did they gain that silly name what exactly does it mean?

The name is European in origin. Highways in England, Ireland, France and Sweden are known as Trunk Roads. The 'trunk' name is much less common in the United States with only three states, all in the upper Midwest, that call their highways Trunk Lines - Michigan, Wisconsin and Minnesota.